May 26, 1911] 



SCIENCE 



825 



SPECIAL ASTICLES 



BIOLOGICAL CONCLUSIONS DRAWN FROM THE 



STUDY OF THE TITANOTHERES ^ 



The chief object of this comraunication is 

 to point out a possible harmony between the 

 " continuity " and. " discontinuity " theories of 

 the phenomena of development. 



The titanotheres, as an extinct family of 

 mammals extending from the summit of the 

 Lower Eocene to the sunmiit of Lower Oligo- 

 cene times, offer an exceptional opportunity 

 for the solution of the two chief modern ques- 

 tions of evolution: first, the mode of trans- 

 formation of existing characters; second, the 

 mode of origin of new characters. The ma- 

 terial available is now the most complete of 

 any extinct family of mammals, embracing 

 several nearly continuous series which branch 

 out into a large number of phyla, some of 

 which may be carried through all the phases 

 of transformation. The investigation has 

 been carried on with the aid of Dr. W. K. 

 Gregory, and is marked by the introduction of 

 a very exact system of measurements, whereby 

 the various kinds of transformation may be 

 studied in numerical terms. This exhaustive 

 research appears to have a significant bearing 

 upon the diverse theories of transformation 

 entertained by the two modern schools of 

 thought, the zoological and botanical, and the 

 paleontological. 



From the time of Waagen in 1869, who in- 

 troduced the term " mutation " for the stages 

 of the continuous development of certain in- 

 conspicuous but genetically important charac- 

 ters in Ammonites, the idea of continuity has 

 become the established law among paleontolo- 

 gists. Neumayr added the conception of 

 " Mutationsrichtung," that is, of trend or 

 direction of development. The researches of 

 Hyatt and Beecher were directed rather to the 

 phenomena of recapitulation than specifically 

 to the phenomenon observed by Waagen. Their 

 writings, however, bring volumes of testimony 

 to the law of continuity of development. This 



' Abstract of paper presented at the meeting of 

 the National Academy of Sciences, Washington, 

 April 19, 1911. Based upon the author's mono- 

 graph "The Titanotheres," in preparation for 

 the TJ. S. Geological Survey. 



law has been established among vertebrates as 

 well as invertebrates. The nature of the evi- 

 dence presented to the paleontologist is en- 

 tirely different from that presented to the 

 zoologist; as an observer the former is prac- 

 tically immortal, that is, his range of ob- 

 servation, where it is possible to assemble 

 continuous series of organisms, extends over 

 enormous periods of time in contrast to the 

 fleeting and essentially mortal glimpses which 

 the botanist and zoologist may obtain of the 

 fundamental processes of transformation. 



Over against the idea of continuity, of defi- 

 nite development, and of certain trends of 

 evolution, there have been developed among 

 the students of living animals and plants 

 (Bateson, de Vries and others) the notion of 

 discontinuity and of order or orderly develop- 

 ment produced only by selection. To this 

 discontinuity de Vries has unfortunately ap- 

 plied the same term " mutation " which was 

 introduced into biological literature by 

 Waagen with entirely different significance; 

 the name saltation should be attached to the 

 de Vries hypothesis; until this is done we 

 must speak of " mutations of Waagen " and 

 "mutations of de Vries." The discontinuity 

 conception has been strengthened rather than 

 'weakened by the wonderful revelations of 

 Mendelian heredity, including the doctrine of 

 unit characters and of " alternate inherit- 

 ance." 



The only tenet which the " continuous " 

 and " discontinuous " schools of thought hold 

 in common, or rather have reached in com- 

 mon, is that germinal evolution is the chief 

 phenomenon upon which our attention must 

 be concentrated. In the present communica- 

 tion the phenomena resulting from somatic 

 changes or ontogeny, from environment, and 

 from selection may be left out of considera- 

 tion, and we may direct our thought solely 

 and exclusively upon germinal evolution as it 

 is displayed in the origin of new characters 

 and in the transformation of existing char- 

 acters in the titanotheres. 



In all the sequence of the titanotheres only 

 four kinds of change are observed: (1) In- 

 crease of size. This happens to be an almost 



